The present invention relates to a detector of a fourier transform infrared spectrometer (FTIR) and, more particularly, a detector using a pyroelectric detector.
The conventional pyroelectric detector is typically a pyroelectric detector of TGS (triglycine sulphide), a DTGS pyroelectric detector of the type in which the hydrogen atom of the TGS pyroelectric detector is replaced by the heavy hydrogen atom, or a DLATGS pyroelectric detector in which L-alanine is doped with the DTGS pyroelectric detector. These pyroelectric detectors have low Curie temperatures. For example, the Curie temperature of the TGS pyroelectric detector is about 49.degree. C. and the Curie temperatures of the DTGS and the DLATGS pyroelectric temperatures are about 62.degree. C. Hence, the figures of merit of the pyroelectric is strongly depend upon the temperature. FIG. 11 shows an example of temperature dependence of the figure of merit in the pyroelectric detector. The data of FIG. 11 are given by the DLATGS pyroelectric detector produced by British Philips under the trade name of RPY104. In FIG. 11, Rv represents sensitivity, NEP represents Noises Equivalent Power, and D* is a figure of merit given by Rv and NEP, totally, indicating a S/N ratio. The data of the figure of merit are normalized based on the value at 25.degree. C. When this DLATGS pyroelectric detector is used, in order to give priority to the stability in the vertical axis of the spectra, it is preferable that the detector should be operated in the range of about 30.degree.-35.degree. C., where at which the sensitivity Rv is rather constant. On the other hand, to obtain the maximum S/N ratio, the detector should be used at about 60.degree. C.
Since the pyroelectric detector shows strong temperature dependence on the figure of merit, a temperature control mechanism should be provided for keeping the temperature of the pyroelectric detector constant whether it is used at room temperature or a higher temperature. Such a temperature control mechanism comprises a thermoelectric cooling element such as a Peltier element close to the pyroelectric detector. The temperature control operation in the conventional pyroelectric detector is done without being synchronized with the data collection by the pyroelectric detector. The input impedance of the pyroelectric detector is high in the range of 10.sup.10 .OMEGA.. Therefore, when a power supply to the thermoelectric cooling element close to the pyroelectric detector is started or shut off, the radiation noise of the electromagnetic wave affects the pyroelectric detector, so that the apparent S/N ratio of the detection signals suffers.